3-D volume imaging has proved to be a valuable diagnostic tool that offers significant advantages over earlier 2-D radiographic imaging techniques for evaluating the condition of internal structures and organs. 3-D imaging of a patient or other subject has been made possible by a number of advancements, including the development of high-speed imaging detectors, such as digital radiography (DR) detectors that enable multiple images to be taken in rapid succession.
Cone beam computed tomography (CBCT) or cone beam CT technology offers considerable promise as one type of diagnostic tool for providing 3-D volume images. Cone beam CT systems capture volume data sets by using a high frame rate flat panel digital radiography (DR) detector and an x-ray source, typically affixed to a gantry that revolves about the object to be imaged, directing, from various points along its orbit around the subject, a divergent cone beam of x-rays toward the subject. The CBCT system captures projection images throughout the source-detector orbit, for example, with one 2-D projection image at every degree of rotation. The projections are then reconstructed into a 3D volume image using various techniques. Among the most common methods for reconstructing the 3-D volume image are filtered back projection approaches.
Although 3-D images of diagnostic quality can be generated using CBCT systems and technology, there are technical challenges. For example, there can be a limited range of angular revolution of the x-ray source and detector with respect to the subject. A full 360 degree orbit is typically used for conventional CBCT imaging though sufficient information for image reconstruction can be obtained with a scan range that just exceeds 180 degrees by the angle of the cone beam itself, for example. However, sometimes it can be difficult to obtain much more than about 180 degree orbit for imaging the knee or other joints and other applications. Even with increased sampling resolution, this angular constraint limits how well a volume image can be reconstructed from its set of 2-D projection images, particularly where there is truncation in one or more of the 2-D projection images.
Image detector sizing for imaging of knee and joints can also be a problem. In some cases, a tradeoff must be made between using a larger image detector that can capture the full image of the subject from any angle, but has additional size and bulk, and a smaller, more portable detector that can be orbited more easily about the subject, but may not be capable of obtaining the complete image at every angle, resulting in image truncation.
Various methods have been used for compensating for image truncation in the projection images captured as part of the CBCT sequence. However, these earlier methods are hampered by problems related to computation efficiency and accuracy of reconstruction of the object that lies within the field of view (FOV). Other problems with known methods can include filter response anomalies and artifacts in the reconstructed image.
Thus, there is a need for improved truncation processing for CBCT images.